The Curious Case of Methane on Mars

Today NASA held a press conference to announce the results of two papers released in Science this week: One on atmospheric methane, and one on organics in the soil, both from Curiosity rover data.

The history of methane detections on Mars is a tumultuous one. In 2004, the European Space Agency announced that Mars Express had detected small amounts of methane in the martian atmosphere. This was exciting news because on Earth, 95% of methane comes from biological sources—mostly microbes and cows, but we’re pretty sure there are no cows on Mars.

Methane should break down in the martian atmosphere over the course of about 300 years. So, a source to replenish the methane is required. There also appeared to be geographic and temporal variability in the methane levels. Ground-based measurements from Earth by a team led by Michael Mumma of NASA Goddard in 2003 and 2006 also pointed to localized methane release that varied over time, although these results were not published until 2009. However, the results from both Mumma and Mars Express were somewhat controversial among the scientific community. Why would the methane not be equally mixed throughout the atmosphere? Where could it possibly be coming from? And were the spectroscopic detections of methane really strong enough to be distinguished from noise?

The Curiosity rover was sent to Mars in 2012, equipped with a laser to look for this methane. In 2013, NASA announced that Curiosity did not detect methane at levels that the tunable laser spectrometer (TLS) could measure. The lowest amounts the TLS is capable of measuring are six times lower than the previously reported estimates for methane concentrations in the atmosphere, so this seemed to be a bit of a nail in the coffin for martian methane.

One of Curiosity’s iconic “selfies” on Mars, taken with the MAHLI camera at the end of its robotic arm. Dozens of images are stitched together to create these selfies, which removes the robotic arm from view. Credit: NASA/JPL-Caltech/MSSS

But time was the key. Over the next two years, Curiosity detected both background levels of atmospheric methane and a 60-sol (martian day) period where big “burps” were observed, causing the methane levels to spike. There was no easy way to explain these burps. They seemed to be too large to come from interplanetary dust or comets, two hypotheses proposed for the methane source. Methane can be produced when UV radiation interacts with organic material, which we know exists in the martian soil, but this wouldn’t explain the seasonal variations.

The newly-released methane results now cover a 3-Mars-year period and confirm strong, repeatable seasonal variations. Methane levels in the atmosphere peak in late southern hemisphere summer into autumn. According to the scientists behind these results, this suggests the methane is slowly being released from underground reservoirs. But that doesn’t answer the question of how the methane is actually being produced. Remember: Since the lifetime of methane in the martian atmosphere is so short, it must be continually produced for us to see it in the concentrations we do.

The methane signal detected by Curiosity has been observed for nearly three Martian years (nearly six Earth years), peaking each summer. Credit: NASA/JPL-Caltech

This leaves two main potential candidates for the methane source: Geological and/or biological. Both have big implications for Mars.

A biological source of methane is of course the most sensational. As I mentioned before, most of Earth’s atmospheric methane is produced by microbes. So, perhaps methanogens—microbes that “eat” carbon dioxide (which makes up most of Mars’ atmosphere) and produce methane—exist in the martian subsurface where they are protected from radiation.

The probably more likely, and less exciting from a headline standpoint, is geologic activity. On Earth, methane can be released through a process called serpentinization. The TL;DR description of serpentinization is that it requires water interacting with rocks. Essentially the thought is that serpentinization may be occurring at depth, producing methane that is then released through cracks up to the surface.

This illustration portrays possible ways that methane might be added to Mars’ atmosphere (sources) and removed from the atmosphere (sinks). Credit: NASA/JPL-Caltech/SAM-GSFC/University of Michigan

To have liquid water interacting with rocks on Mars, that means you need a heat source. Until recently, scientists thought Mars’ core was solid and we’d seen no evidence for volcanic activity younger than about 100 million years ago. But the Mars Atmosphere and Volatile Evolution mission (MAVEN), which arrived in 2014, observed auroral activity at Mars. And this requires a magnetic field. So, perhaps Mars isn’t geologically dead on the inside after all! The InSight lander, on its way to the Red Planet right now, will help to answer this question. Stay tuned!

On Earth, generally heat + liquid water = life. So, this might not be an either/or scenario on Mars. Perhaps geology and biology both play roles in the story of martian methane. With the data we have right now though, we can’t say for sure one way or the other.

Aurora on Mars observed by the MAVEN spacecraft in 2017. Credit: NASA/GSFC/SwRI

The conclusion of this article might be wholly unsatisfying: We still don’t know exactly where methane on Mars is coming from. It will take more time and data to help pin down the answer. India’s Mars Orbiter Mission (MOM), which arrived at Mars in 2014, is equipped with a sensor to investigate atmospheric methane, but a design flaw with the instrument likely means it won’t be able to collect definitive data. Meanwhile, Europe’s ExoMars Trace Gas Orbiter (TGO) just settled into its science orbit in April of this year and is now collecting data to help understand the mystery of martian methane. Putting all of the pieces together from the ground and from orbit will help us paint the larger picture of where this methane is coming from.